The T-box transcription factor TBX1 was recently identified as the gene responsible for the etiology of 22q11 deletion syndrome (22q11DS). Conductive hearing loss occurs in a majority of patients with this syndrome, while sensorineural deafness has also been reported in some cases. Mutation of the murine orthologue, Tbx1, has proven to be an excellent model for this disease. Mice null for Tbx1 fail to develop an outer and middle ear, while the inner ear never develops beyond the otic vesicle stage. In addition, the cochleovestibular ganglion (CVG), which forms from the otic vesicle, is duplicated in Tbx1 null embryos. Epithelial and mesenchymal interactions are thought to play an important role in the development of the both the cochlea and vestibular system, however many of the signaling pathways that control inner ear development are not known. Tbx1 is expressed both in the otic vesicle epithelium and the surrounding periotic mesenchyme (POM). We hypothesize that Tbx1 signaling in the POM is required for cochlear development via a genetic interaction with the transcription factor Brn4. For Specific Aim 1, Tbx1 will be inactivated in the POM using the Cre/loxP system to create a conditional mouse mutant, enabling the isolation of the role of Tbx1 in this tissue. In Specific Aim 2, the interaction between Brn4 and Tbx1 in the POM will be tested by generating mice mutant for both genes. Heterozygous null mutations in either Tbx1 or Brn4 do not produce inner ear malformation. Compound heterozygous mice harboring mutations for both genes will be analyzed for inner ear defects. Achievement of these specific aims will provide a further insight into the role of Tbx1 in inner ear development and the genetic pathways in which it functions. Deafness is a major public health issue, and the genetic causes of deafness are still poorly understood. TBX1 is the gene responsible for many of the symptoms of 22q11 DS, including hearing loss. Understanding of how this gene contributes to normal ear development is crucial to gaining a better knowledge of the causes of congenital deafness and will lead to improved detection of this disease. [unreadable] [unreadable] [unreadable]